Instrumental deviations, from Beer’s law

Stray radiation is the second contribution to instrumental deviations from Beer s law. Stray radiation arises from imperfections within the wavelength selector... 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

The optimum working range for percent transmittance (to avoid instrumental deviations from Beer s law) is between 15 and 80%, which corresponds to an absorbance range of 0.10 to 0.82. 

No ordinary monochromator is capable of yielding a band of radiation as naiTOW as the width of an atomic absorption line (0.002 to 0.005 nm). As a result, the use of radiation that has been isolated from a continuum source by a monochromator inevitably causes instrumental departures from Beer s law (see the discussion of instrument deviations from Beer s law in Section 24C-3). In addition, since the fraction of radiation absorbed from such a beam is small, the detector receives a signal that is less attenuated (that is, P —> Pq) nd the sensitivity of the measurement is reduced. This effect is illustrated by the lower curve in Figure 24-17 (page 733). 

Instrumental deviations from Beer s law Departures from linearity between absorbance and concentration that are attributable to the measuring device. 

With IR radiation, instrumental deviations from Beer s law arc more common than with ultraviolet and visible wavelengths because IR absorption bands are... 

Deviations from Beer s law are in evidence when the Beer s law plot is not linear. This is probably most often observed at the higher concentrations of the analyte, as indicated in Figure 8.10. Such deviations can be either chemical or instrumental. 

At a constant cell path length, Beer s law shows that the absorbance of radiation through a medium is proportional to the concentration of the solute. Beer s law is strictly valid only for monochromatic radiation. Stray light (i.e., scattered radiation), which reaches the detector without having passed through the desired beam path, molecular interactions such as hydrogen bonding, which varies with the sample concentration, and other instrumental factors such as slit width, all affect molar absorptivity and result in some deviations from Beer s law. For an accurate analysis of the concentration of an unknown sample, it is usually necessary to first create a calibration curve from standard... 

Deviations from Beer s law caused by instrumental parameters are discussed later. 

Generally, the better the instrument, the less liJcely are deviations from Beer s law due to polychromatic radiation. 

Describe the difference between real deviations from Beer s law and those due to instrumental or chemical factors. 

Deviations from Beer s law may result from either chemical reasons connected with the sample, or physical ones connected with the instruments involved [6-8]. In the former case... 

Deviations from Beer s law may also arise from insufficient quality of measuring instruments, mainly from the use of non-ideal monochromatic light, improper width of the spectral band, or scattering of radiation. The detector signal should be proportional, over a wide range, to the intensity of the radiation recorded. 

It cannot always be assumed that Beer s law will apply, that is, that a linear plot of absorbance versus concentration will occur. Deviations from Beer s law occur as the result of chemical and instrumental factors. Most deviations from Beer s law are really only apparent deviations because if the factors causing nonlinearity are accounted for, the true or corrected absorbance-versus-concentration curve will be linear. True deviations from Beer s law will occur when the concentration is so high that the index of refraction of the solution is changed from that of the blank. A similar situation would apply for mixtures of organic solvents with water, and so the blank solvent composition should closely match that of the sample. The solvent may also have an effect on the absorptivity of the analyte. 

Deviations from Beer s law may be due to instrumental factors or to chemical factors. These deviations may result in an upward curvature (positive deviation) or in a downward curvature (negative deviation), as shown in Figure 7.5. A check on instrumental factors can be made by plotting absorbance versus cell length at a constant concentration this plot will be linear if the instrument is performing satisfactorily. Deviations arising from chemical factors are observed only when concentrations are changed. 

Thus, there is a negative deviation from Beer s law. Errors due to stray light are more commonly found near the wavelength limits of the instrument components. Many reports of spectra in the UV region below 220 nm should be carefully checked, since false peaks have been reported. Visible radiation usually presents the most serious stray-light problem for ultraviolet-visible spectrophotometers, because both the spectral radiance of most visible sources and the spectral response of most detectors to visible radiation are high. 

Instrumentation limitations may also result In deviations from Beer s law. One of the meiln causes here is tmpetfect monochromacy. Let us see what this means. 

According to Beer s law, a calibration curve of absorbance versus the concentration of analyte in a series of standard solutions should be a straight line with an intercept of 0 and a slope of ab or eb. In many cases, however, calibration curves are found to be nonlinear (Figure 10.22). Deviations from linearity are divided into three categories fundamental, chemical, and instrumental. 

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